1. Field of the Invention
The present invention relates to an optical amplifier suitable for use in amplifying optical signals, particularly those in newly available bands, and for use in a wavelength-division multiplexing optical transmission system, in which multiple optical signals at different wavelengths are combined to be transmitted through one optical fiber. The invention also relates to a method for compensating for gain tilts (variations) across a range of wavelengths occurring in the present optical amplifier.
2. Description of the Related Art
In a wavelength-division multiplexing optical transmission system (optical transmission system), which transmits wavelength-division multiplexed optical signals at different wavelengths, an optical fiber transmission path reveals low (about 0.3 dB/km or smaller) transmission loss in a limited range of wavelengths from 1450 to 1650 nm. Various types of optical amplifiers have thus been developed to amplify optical signals in this range.
A Raman amplifier (discrete Raman amplifier, Raman fiberamplifier; RFA) is a commercially available example. It uses the Raman effect, which is a nonlinear effect in optical fibers, to amplify optical signals. Raman amplifiers employ optical fibers having high nonlinearity, such as dispersion-compensating fibers (DCFs) and highly nonlinear fibers (HNLFs).
Such a Raman amplifier is advantageous in that it can amplify optical signals at arbitrary wavelengths by selecting the wavelength of a pump beam, whereas it has a disadvantage of limited output power caused by nonlinear effects such as four-wave mixing. Additionally, when the Raman amplifier amplifies optical signals in the S-band, the wavelength of the pump beam falls in a range in which optical fibers exhibit greater loss, so that the amplification efficiency is impaired. Moreover, since gain per unit length is small, the Raman amplifier requires several to several tens of kilometers of fiber (optical amplifier medium) for optical amplification, making downsizing difficult.
Another type of amplifier is commercially available which amplifies optical signals by stimulated emission. The stimulated emission is caused by a population inversion in the energy formed by excitation within an optical amplifying fiber (optical amplifier medium). The following are examples.
As an optical fiber amplifier for amplifying optical signals at wavelengths of 1530 to 1560 nm {the 1550-nm band, the C-band (Conventional-wavelength band), an optical fiber amplifier (erbium-doped fiber amplifier; EDFA) that employs an erbium-doped fiber (EDF) has been developed. In addition, another type of erbium-doped fiber amplifier (gain-shifted erbium-doped fiber amplifier; GS-EDFA) has also been developed for amplifying optical signals at wavelengths of 1570 to 1600 nm {the 1580-nm band, which is the L-band (Longer-wavelength band)}.
As an optical fiber amplifier for amplifying optical signals at wavelengths of 1450 to 1490 nm (the S+-band), there has already been provided an optical amplifier that employs a thulium-doped fiber (thulium-doped fluoride-based fiber amplifier; TDFA). The TDFA is a kind of rare-earth doped fiber amplifier which is advantageous in that it has a high level of output power, and in that it is capable of amplifying optical signals in the S+-band, a range of wavelengths shorter than those of the S-band. The TDFA, however, has a disadvantage of reduced yield because of the use of a fluoride-based fiber, and its reliability is still rather poor.
Further, as a brand-new type of optical fiber amplifier for amplifying optical signals at wavelengths of 1475 to 1510 nm, a gain-shifted thulium-doped fluoride-based fiber amplifier has been under development. Even with such anew optical amplifier, it is still difficult to amplify optical signals at wavelengths of 1510 to 1530 nm, out of a band ranging from 1490 to 1530 nm {the S-band (the shorter-wavelength band)}.
With recent expansion of the market of wavelength-division multiplexing optical transmission systems, their further increment in capacity has long been desired.
Among the foregoing optical fiber amplifiers, an EDFA is a kind of rare-earth doped fiber amplifier which exhibits a good performance at amplifying optical signals in the C-band and the L-band, revealing a high level of output power. Additionally, it is free from the forgoing drawbacks in RFAs and TDFAs.
In an attempt to further increase the capacity of transmission, it has been expected that an EDFA having new optical amplification bands, one (the S-band) from 1490 to 1530 nm and the other (the S+-band) from 1450 to 1490 nm, will be developed to expand the available transmission bands.
In order to amplify optical signals in the S-band and the S+-band, the EDFA should employ an erbium-doped fiber made from multicomponent glass, or alternatively, the EDFA should operate in a state where the EDF maintains a high inversion rate in such a manner that gain could be obtained in the S-band and the S+-band.
In the meantime, EDF has higher gain coefficients in the C-band than in the S-band or the S+-band. Amplified spontaneous emission (ASE) in the C-band thus significantly grows, thereby impairing the efficiency of amplification in the S-band and the S+-band. Moreover, gain tilts (variations), gain deviations between the shorter wavelengths and the longer wavelengths, are increased significantly in those bands.
In view of these, for accomplishing amplification of optical signals in the S-band and the S+-band using an EDFA, the EDFA should operate at a high inversion rate to improve the amplification efficiency in the S-band and the S+-band, and its gain characteristic should be flattened across a range of wavelengths.
To satisfy these requirements, the characteristics of optical filters should be optimized, the length of a single EDF should be optimized, or the number of EDFs should be optimized. Most importantly, the characteristics of optical filters should be optimized.
Further, even if this optimization achieves amplification of optical signals in the S-band and the S+-band, it is still necessary to compensate with certainty for gain tilts caused by manufacturing errors of the optical filters or others.
Japanese Patent Application Publication No. 2000-124529 (United States Patent Application Publication No. US/2002/0001124) discloses an optical amplifier that includes multiple optical fiber amplifiers. In the application, there is given a description as if multiple optical filters alone, each of which is interposed between the individual optical fiber amplifiers, enable amplification of optical signals in the S-band. Only when there are such multiple EDFs and multiple optical filters each interposing therebetween, however, does it become practically impossible to amplify optical signals in the S-band or the S+-band.
Meanwhile, though EDFAs have already achieved amplification of optical signals in the L-band, from 1570 to 1600 nm, there are still various problems caused by EDFs with the lengths of up to 50 m. One example of such problems is that the output power is significantly decreased when a longer-wavelength optical signal is input alone, in comparison with the case in which both a longer-wavelength optical signal and a shorter-wavelength optical signal are input together. Similarly, the increased length of EDFs might also be a problem in an attempt to amplify optical signals in the L+-band, from 1610 to 1650 nm, by an EDFA.